AWS Encryption SDK 1.3.0 release

Major

• Added cryptographic materials managers as a concept
• Added data key caching
• Moved to deterministic IV generation

Minor

• Added changelog
• Fixed attrs usage to provide consistent behavior with 16.3.0 and 17.x
• Fixed performance bug which caused KDF calculations to be performed too frequently
• Removed line_length as a configurable parameter of EncryptingStream and DecryptingStream objects to simplify class APIs after it was found in further testing to have no measurable impact on performance
• Added deterministic length eliptic curve signature generation
• Added support for calculating ciphertext message length from header
• Migrated README from md to rst